Flat-panel display devices, for example plasma, liquid crystal and electroluminescent (EL) displays have been known for some years and are widely used in electronic devices to display information and images. EL display devices rely upon thin-film layers of materials coated upon a substrate, and include organic, inorganic and hybrid inorganic-organic light-emitting diodes (LEDs). The thin-film layers of materials can include, for example, organic materials, inorganic materials such as quantum dots, fused inorganic nano-particles; and electrodes, conductors, zinc oxide, and silicon electronic components as are known and taught in the LED art. Such devices employ both active-matrix and passive-matrix control schemes and can employ a plurality of light-emitting elements. The light-emitting elements are typically arranged in two-dimensional arrays with a row and a column address for each light-emitting element and having a data value associated with each light-emitting element to emit light at a brightness corresponding to the associated data value.
Typical large-format displays (e.g. having a diagonal of greater than 12 to 20 inches) employ hydrogenated amorphous silicon thin-film transistors (aSi-TFTs) formed on a substrate to drive the pixels in such large-format displays. The manufacturing process conventionally employed to form aSi-TFTs typically produces TFTs whose characteristics vary spatially over the surface of the substrate. However, the local aSi-TFT variation is typically relatively small so that neighboring TFTs will have similar characteristics while TFTs spaced further away will differ more. In contrast, smaller-format displays, (e.g. having a diagonal of less than 12-20 inches) generally use polysilicon, although amorphous silicon may be used as well, containing small crystalline structures that improve the mobility of the silicon and, hence, its performance. The crystals are typically formed by heating the surface of an amorphous silicon layer with a laser, for example an excimer laser. Exemplary patent application, US2006/0009017 filed by Scmbommatsu et al on 17 Jun. 2005, entitled “Method Of Crystallizing Semiconductor Film And Method Of Manufacturing Display Device” describes a method of uniformly crystallizing a semiconductor film through scanning with pulse lasers. However, this approach may lead to crystalline granules with variable performance so that neighboring TFTs can have quite different performance characteristics that are readily visible in a display using such polysilicon TFTs. Moreover, the annealing process is expensive. Hence, amorphous silicon thin-film transistors are characterized by large-scale non-uniformity and relatively low mobility, while polysilicon thin-film transistors are characterized by small-scale non-uniformity, relatively higher mobility, and higher cost.
Moreover, as described in “Threshold Voltage Instability Of Amorphous Silicon Thin-Film Transistors Under Constant Current Stress” by Jahinuzzaman et al in Applied Physics Letters 87, 023502 (2005), the aSi-TFTs exhibit a metastable shift in threshold voltage when subjected to prolonged gate bias. This shift is not significant in traditional display devices such as LCDs, because the current required to switch the liquid crystals in LCD display is relatively small. However, for LED applications, much larger currents must be switched by the amorphous silicon thin-film transistors (aSi-TFT) circuits to drive the electroluminescent materials to emit light. Thus, electroluminescent displays employing aSi-TFT circuits are expected to exhibit a significant voltage threshold shift as they are used. This voltage shift may result in decreased dynamic range and image artifacts. Moreover, the organic materials in organic EL (OLED) and hybrid EL devices also deteriorate in relation to the integrated current density passed through them over time, so that their efficiency drops while their resistance to current increases.
One approach to avoiding the problem of voltage threshold shift in TFT circuits is to employ circuit designs whose performance is relatively constant in the presence of such voltage shifts. For example, US2005/0269959 filed by Uchino et al, Dec. 8, 2005, entitled “Pixel Circuit, Active Matrix Apparatus And Display Apparatus” describes a pixel circuit having a function of compensating for characteristic variation of an electro-optical element and threshold voltage variation of a transistor. The pixel circuit includes an electro-optical element, a holding capacitor, and five N-channel thin-film transistors including a sampling transistor, a drive transistor, a switching transistor, and first and second detection transistors. Alternative circuit designs employ current-mirror driving circuits that reduce susceptibility to transistor performance. For example, US2005/0180083 filed by Takahara et al., Aug. 15, 2005 entitled “Drive Circuit For El Display Panel” describes such a circuit. However, such circuits are typically much larger and more complex than the two-transistor, single capacitor circuits often employed, thereby reducing the area on a display available for emitting light and decreasing the display lifetime.
Other methods useful for aSi-TFTs rely upon reversing or slowing the threshold-voltage shift. For example, US2004/0001037 filed Jan. 1, 2004 by Tsujimura et al., entitled “Organic Light-Emitting Diode Display” describes a technique to reduce the rate of increase in threshold voltage, i.e. degradation, of an amorphous silicon TFT driving an OLED. However, such schemes typically require complex additional circuitry, thereby reducing the geographical area on a display available for emitting light and decreasing the display lifetime.
JP 2002-278514 by Numeo Koji, published Sep. 27, 2002, describes a method in which a prescribed voltage is applied to organic EL elements by a current-measuring circuit and the current flows are measured; and a temperature measurement circuit estimates the temperature of the organic EL elements. A comparison is made with the voltage value applied to the elements, the flow of current values and the estimated temperature, the changes due to aging of similarly constituted elements determined beforehand, the changes due to aging in the current-luminance characteristics and the temperature at the time of the characteristics measurements for estimating the current-luminance characteristics of the elements. Then, the total sum of the amount of currents being supplied to the elements in the interval during which display data are displayed, is changed so as to obtain the luminance that is to be originally displayed, based on the estimated values of the current-luminance characteristics, the values of the current flowing in the elements, and the display data. This design is not useful for dealing with non-uniformities between different light-emitting elements or will require excessive measurement time.
It is known in the prior art to measure the performance of each pixel in a display and then to correct for the performance of the pixel to provide a more uniform output across the display. U.S. Pat. No. 6,081,073 entitled “Matrix Display with Matched Solid-State Pixels” by Salam and issued Jun. 27, 2000 describes a display matrix with a process and control means for reducing brightness variations in the pixels. This patent describes the use of a linear scaling method for each pixel based on a ratio between the brightness of the weakest pixel in the display and the brightness of each pixel. U.S. Pat. No. 6,473,065 entitled “Methods Of Improving Display Uniformity Of Organic Tight Emitting Displays By Calibrating Individual Pixel” by Fan, issued Oct. 29, 2002 describes methods of improving the display uniformity of an OLED. In order to improve the display uniformity of an OLED, the display characteristics of all organic-light-emitting-elements are measured, and calibration parameters for each organic-light-emitting-element are obtained from the measured display characteristics of the corresponding organic-light-emitting-element. The calibration parameters of each organic-light-emitting-element are stored in a calibration memory. The technique uses a combination of look-up tables and calculation circuitry to implement uniformity correction. However, these approaches require the performance measurement of each light-emitting element in the display. While this may be practical in a factory, it is not useful to accommodate changes in the device performance as it is used, since the measurements may take a considerable amount of time and therefore decrease the usability of the display during that time, discommoding the viewer of the display. Applicants have also determined through experimentation that, despite measures taken to reduce the instrumentation noise in the light-emitting element measurements, it may be difficult to consistently and accurately measure the light output from each of the light-emitting elements.
There is a need, therefore, for an improved method of providing uniformity in an active-matrix EL display having amorphous silicon thin-film transistors that overcomes these objections.